I don't know what Judy Collins published, but the usual tune The Great Selkie is sung to these days was written by James Waters, an academic at Harvard, in the early 1950s. There is a traditional tune (collected just before WW2 and printed in Bronson) but it's rather less memorable. Here is Joan Baez singing the Waters tune:

http://www.youtube.com/watch?v=6zZy2Q3QY0Q

We've had a few threads about this one.

I assumed that Sue wanted to avoid tunes that were associated with any particular text. There are situations when you'd want that.

How could anybody say that a certain slow air has never been sung?

Quite easily, with most fiddle slow airs. They tend to be complicated and hard to sing, so if anybody did manage to set words to one, it would be an achievement people would remember (even if, as with some of Burns's efforts, the resulting song was something only an opera singer would try).

It still goes on. There is a song by Karine Polwart using a recent fiddle tune, Ivan Drever's "Leaving Stoer". I forget the song title. It has the same range as "Danny Boy", but... well, you try singing it. The words are on a worthwhile subject but that song's going nowhere.